Struct

Struct #

Struct is a way to group related data together, and It’s a typed collection of fields.


Let’s see it declares Program class and Main method.

We haven’t declared an entrypoint of class and a Main method in the previous examples, because the compiler automatically generates those for us.

In this example, we explicitly declare a Program class to define an Employee struct at the top-level of a file.

public struct Employee {
    public string name;
    public int id;
    public int age;
};

class Program {
    public static void Main(string[] args) {
        var emp = new Employee();
        emp.name = "John";
        emp.id = 1;
        emp.age = 23;

        Console.WriteLine($"name: {emp.name}"); // name: John
        Console.WriteLine($"id: {emp.id}");     // id: 1
        Console.WriteLine($"age: {emp.age}");   // age: 23
    }
}

Employee struct has three fields: name, id, and age. Each fields has a type, a name and an indicator of its accessibility. 

public struct Employee {
    public string name;
    public int id;
    public int age;
};

We can define a method in structs. 

public struct Employee {
    public string name;
    public int id;
    public int age;

    public string toString() {
        return $"ID: {id}, name: {name}, age: {age}";
    }
};

We can use the new operator to create an instance of a struct or declare a variable of the struct type. 

// declare with new opperator
var emp1 = new Employee();

// declare a variable with struct type.
Employee emp2;

emp1.name = "John";
emp1.id = 1;
emp1.age = 23;

emp2.name = "Mark";
emp2.id = 2;
emp2.age = 25;

Console.WriteLine(emp1.toString());
Console.WriteLine(emp2.toString());

The all fields of created instance with new operator is initialized with default values.

On the other hand, the variable with type doesn’t initialize their fields. Therefore, we need to manually put values to each fields.

This example shows that It’s not allowed to use a variable without initialization.

public struct Employee {
    public string name;
    public int id;
    public int age;

    public string toString() {
        return $"ID: {id}, name: {name}, age: {age}";
    }
};

Employee emp;

// Local variable 'emp' might not be initialized before accessing
Console.WriteLine(emp.toString());

You can’t access a field of a struct if it has private access modifier. 

struct Employee {
    public string name;
    public int id;
    private int age;
};

var emp = new Employee();
emp.name = "John";
emp.id = 1;
emp.age = 23; // error: 'Employee.age' is inaccessible due to its protection level

The private fields are accessible only within the body of the struct in which they are declared. 

struct Employee {
    public string name;
    public int id;
    private int age;

    public void setAge(int age) {
        this.age = age;
    }
};

var emp = new Employee();
emp.name = "John";
emp.id = 1;
emp.setAge(23);

Let’s define a paramiterized constructor inside a User struct. 

public struct User {
    public string Id;
    public string Name;
    
    public User(string id, string name) {
        this.Id = id;
        this.Name = name;
    }
}

class Program {
    public static void Main(string[] args) {
        var user = new User("scalalang", "David");
        Console.WriteLine(user.Id);
        Console.WriteLine(user.Name);
    }
}

Struct is a value type. #

Structs are a value type, It means that all values of fields are copied when the struct is assigned to a new variable or passed as a parameter. 

public struct User {...}

var user = new User("scalalang", "David");
var user2 = user;
user.Id = "I-love-csharp";
Console.WriteLine(user.Id);  // I-love-csharp
Console.WriteLine(user2.Id); // scalalang